A semiconductor element (chip) used as a microprocessor unit (MPU) or the like is electrically connected and fixed (for example, flip-chip bonded) onto a wiring board (package). Since the semiconductor element is brought to an extremely high temperature state during its operation, the temperature of the semiconductor element has to be forcibly reduced. Otherwise, the semiconductor element cannot exhibit its performance, and even worse, may break down in some cases.
For this reason, a heat-radiating component (for example, metal heat spreader) for releasing heat generated by the semiconductor element into the atmosphere is mounted on the semiconductor element. This secures a route through which the heat generated by the semiconductor element is released to the outside. In this configuration, the semiconductor element and the heat spreader are thermally bonded to each other with a material called a thermal interface material (TIM) interposed therebetween. Specifically, the unevenness on each of the surfaces of the semiconductor element and the heat spreader is absorbed by interposing the TIM, thereby reducing the contact thermal resistance between the surfaces. This allows smooth heat conduction from the semiconductor element to the heat spreader. Such thermal interface material (TIM) includes one formed of a high thermal conductivity material such as silicone grease, indium, or graphite into a sheet shape by using a resin binder.
FIG. 5 illustrates, as an example of the related art, a configuration of a semiconductor device using a heat spreader. In FIG. 5, reference numeral 1 denotes a wiring board (package), reference numeral 2 denotes pins serving as external connection terminals of the package 1, reference numeral 3 denotes a semiconductor element (chip which generates large amount of heat, such as CPU) mounted on the package 1, reference numeral 4 denotes chip capacitors for decoupling mounted around an mounting area of the chip 3, and reference numeral 5 denotes a heat spreader. The heat spreader 5 has a structure in which its main portion is formed in a plate shape (plate-like portion 5a), and a foot portion 5b is integrally formed on the periphery of the plate-like portion 5a. The heat spreader 5 is thermally bonded to the chip 3 with a thermal interface material (TIM) 6 interposed between the plate-like portion 5a and the chip 3. In addition, the heat spreader 5 is mechanically connected to the package 1 with a sealant 7 interposed between the foot portion 5b and the package 1. Namely, the heat spreader 5 is mounted in such a manner that the chip capacitors 4 are sealed therein together with the semiconductor chip 3. In addition, a heat sink 8 is mounted on the heat spreader 5.
An example of a technique related to such conventional art is described in Patent document 1 (Japanese Laid-open Patent Publication No. 2003-51573). The Patent document 1 discloses a power module in which a wiring board having a heat generating component electrically connected thereto and a heat sink are connected to each other via a heat-conductive electrically-insulating member (paragraph 0084, FIG. 7). Moreover, as another technique related to the conventional art, there is a semiconductor device described in Patent document 2 (Japanese Laid-open Patent Publication No. 2004-119882). In this semiconductor device, a semiconductor element is flip-chip bonded onto a recessed portion in a multilayer circuit board, passive elements are mounted on the board in an area around the recessed portion, and a heat-radiating member is bonded to a surface of the semiconductor element through a bonding material (paragraph 0027, FIG. 1).
In the configuration of the semiconductor device using the heat spreader illustrated in FIG. 5, the heat spreader 5 is mounted in such a manner that the chip capacitors 4 mounted around the semiconductor chip 3 are also sealed. Accordingly, a distance between a position where the foot portion 5b is fixed to the board 1 through the sealant 7 and a position where the chip 3 is mounted on the board 1 is relatively long. Thus, there is a case where the TIM 6 could not follow the thermal behavior of the board 1 (bending of the board 1 which may occur due to heat generated by the chip 3).
Namely, when heat is applied to the board (package) 1, the package 1 bends outward (downward in the illustrated example) as illustrated in FIG. 5 at the position where the foot portion 5b is fixed to the substrate 1 by the sealant 7. In this state, the contact state of the TIM 6 with the chip 3 or with the heat spreader 5 changes, or the thickness of the TIM 6 changes. Thus, a thermally unstable state occurs. As a result, heat is not conducted smoothly from the chip 3 to the heat spreader 5, and stable heat radiation property cannot be maintained.
In particular, where an inexpensive grease is used as the material of the TIM 6, the TIM 6 may be partially peeled off from the chip 3 or the heat spreader 5 upon bending of the package 1 (in the illustrated example, a state where the TIM 6 is peeled off from the chip 3 is exaggeratedly illustrated). In such case, the heat resistance becomes high in the peeled off area, and thus the TIM 6 cannot exhibit its high heat-conductivity.
Moreover, the heat spreader 5 seals on the package 1, the chip 3 such as CPU which generates large amount of heat, and a portion where the chip 3 is thermally bonded in a recessed portion of the heat spreader 5 is only partial. Thus, hot air tends to be accumulated in the recessed portion of the heat spreader 5. From this point of view as well, there remains room for improving the heat radiating performance.